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Why are true and indicated airspeed different?
So, I understand that True air speed is the motion of aircraft relative to air around it. And, indicated airspeed is speed arising due to dynamic pressure.
Considering a true airspeed of 100 knots, that means the air molecules are striking the aircraft at 100 kt/ vice versa.
Shouldn't this make true and indicated airspeed same?
Also with constant indicated airspeed, why does true airspeed increase with altitude? Shouldn't it remain same?

Air pressure reduces with altitude, so while an individual molecule is striking the the aircraft at 100kt, there are fewer of them. This means there is less pressure placed on whatever surface is being struck.
The difference in indicated speed is simply one of a physical limitation with the air speed indicator - air speed indicators rely on dynamic pressure/ram air to give a readout. Because of that reduction in pressure, the readout begins to diverge from the true, physical speed as air pressure differs from the reference pressure of the gauge.
It would be possible to create an airspeed indicator which adjusts for this, in the same way as the altitude indicator does. However, as it happens, the planes flight characteristics also changes as pressure reduces in much the same way as the air speed indicator does. Because of this, for pilots, the IAS is the important speed outside of navigational purposes.
Hence, all design speeds are written in Indicated Air Speed and can be referred to at any altitude.

But, how can a true airspeed of 187 knots be achieved at 2400 FL with OAT -26, with IAS being 125 kt? I mean, at 187 knots, how are air molecules striking aircraft at a rather lower speed of 125 kt?

@AmanVishwakarma The indicator is not directly measuring the speed at which air molecules strike the aircraft; it's showing a value derived from the dynamic pressure difference measured by the pitot-static system.

@Aman Vishwakarma They're not, the molecules are moving at whatever speed they're moving at. The problem is that the air speed indicator doesn't know what speed they're moving at because it relies on dynamic pressure to create a reading.

@AmanVishwakarma. the probe is not measuring velocity of the molecules, but the force created by their impacts. And force is mass times acceleration. Fewer molecules, lower force.

Think of it this way. Your airspeed indicator is not directly measuring the distance traveled of any object, aircraft nor air molecules, over time. Your airspeed indicator is a pressure sensitive device that measures ram air pressure (dynamic pressure) versus static pressure. It converts that measurement into a meaningful value of airspeed.
That ram air pressure device is dependent on the density, or more precisely, the mass, of the fluid it measures. The less mass there is to measure, the less ram air pressure measured. The Earth’s atmospheric pressure decreases logarithmically with altitude due to the weight and density of air as a fluid. Half of Earth’s atmospheric mass is below 18000 feet MSL. It’s density decreases rapidly from there. If the same pitot tube at the same velocity were placed in water, it would measure a drastically greater IAS due to greater mass of water in the same volume of space. Even if the aircraft remains at the same actual velocity, the IAS would change with the mass (therefore density) of the air entering the pitot tube. This is because there are fewer air molecules In a given volume of air to create the mass.
A real world analogy is the difference between standing in a brisk wind versus standing in a moving current of water. The moving wind would have to be moving at a very fast speed in order to exert enough pressure or force to overcome your inertia. The moving water of the current does not have to be moving nearly as fast at all to do the same.

I very well understood that! I was think of IAS as measure of distance rather than focusing on the pressure component . So can we say that, it's just a measure of density to better understand the landing conditions in low airspeeds?

@AmanVishwakarma - I am not quite sure I understand what you mean by that. Think of the IAS as a way to compare apples to apples (a western idiom) regardless of density altitude. Your Vspeeds (not just the slow ones), will be dependent on the effect of dynamic pressure on the airframe. Your Airspeed Indicator measures dynamic pressure at the pitot tube. Since both the dynamic pressures on the airframe and the pitot tube are at the same density altitude, your Vspeeds will be the same regardless of density altitude. Vne will be Vne by IAS regardless of how TAS changes due to density altitude.

Dynamic pressure is
$$ q = ½\varrho v^2 $$
where $q$ is dynamic pressure, $\varrho$ is density and $v$ is velocity (a.k.a. true airspeed). The important bit here is that it is proportional to density and since density and pressure are closely related and pressure decreases with altitude, so does density
For the intuition of particles hitting the surface, note that pressure is force times area and force is mass times acceleration. So lower density (density is mass per volume) means less force and less pressure for the same velocity.

I would point out $v$ in this equation is true airspeed, and $ \sqrt{\bar{q}} $ is proportional to indicated airspeed.

TL-DR: IAS is not about speed but about dynamic pressure. You can read it as "This is how fast I would need to fly at sea level standard conditions in order to get the same dynamic pressure"

As Jan Hudec pointed out already, indicated airspeed is computed from dynamic
pressure. That itself is computed from total (i.e. stagnation) and static pressure, and the difference between both.
For (lossless) incompressible flow (i.e. anything slower than half the speed of sound, you might remember Bernoulli's equation:
$\frac{1}{2} \rho_\infty v_\infty^2 + P_\infty = P_T$
What is measured in flight is $P_T$ (total pressure, at the stagnation point) and $P_\infty$ (static pressure of undisturbed flow, measured the sides of a pitot-static tube). The difference between both is called dynamic pressure ($q_\infty$), and allows you to compute velocity:
$v_\infty = \sqrt{ \frac{2q}{\rho_\infty}} $
...except you need to know the density. Density mostly follows the ideal gas law, which means that it varies with pressure and temperature, but it also changes with humidity.
In actual aviation, such effects can be taken into account for the purposes of navigation. You can work out the actual airspeed plus windspeed fairly well -- but for the purpose of designing an aircraft, dynamic pressure is actually more important than true airspeed, and also easier to compute. So when designing an aircraft and calculating loads, as well as as minimum and top speed, indicated airspeed is actually more useful than true airspeed, as it scales directly with dynamic pressure, and all aerodynamic loads on an aircraft scale directly with that.
That is why for example take-off, landing and stall speed are quoted as IAS. The important bit is not how fast you're moving but whether you're generating enough dynamic pressure to stay in the air (or take off, and so on...). When you're taking off in Lhasa you need the same IAS as in New York, but you need a higher TAS in Lhasa to compensate for the thinner air.
Now, for compressible flow (very roughly: going faster than half the speed of sound), the incompressible Bernoulli equation is not that accurate anymore. Prandtl-Glauert transformation can kinda make it work a bit further, but really it makes a lot more sense to stop thinking about velocity and use Mach number instead, and the isentropic state change equations. They result in this handy equation to work out velocity:
$v_\infty = M * a = \sqrt{\frac{\gamma-1}{2} \left(\left(\frac{P_T}{P_\infty}\right)^{\frac{\gamma-1}{\gamma}} -1\right)} \sqrt{\gamma R_s T_\infty} $
($\gamma \approx 1.4$ for air, $R_s$ is the specific gas constant, and $T_\infty$ is static temperature)
It's perfectly feasible to use this to work out the true airspeed (given some information on temperature and humidity, and calibration of the probe given that it doesn't quite see undisturbed flow) -- but except for navigation, IAS is still used when designing and flying an aircraft.
Reason: It's all about dynamic pressure. Someone who uses IAS is usually actually interested in dynamic pressure, not how fast the aircraft is moving. Using a velocity rather than a pressure is done out of habit (because for many decades, IAS was the only number available to pilots), and because it actually approaches true airspeed at lower Mach numbers and altitudes (i.e. for take-off and landing), so most pilots already have a "feeling" for those numbers. That means it makes sense to keep using IAS instead of straight-up pressure.
This is why even transonic aircraft have not just take-off, landing and stall speed quoted in IAS, but even top speed is limited not just in terms of Mach number but also IAS. This means: You're fine to fly at Mach 0.85 at 11km altitude but if you need to make an emergency descent you can't stay at M=0.85 all the way down to 2km because the aircraft was not built to deal with the dynamic pressure you'd get in the much thicker air at lower altitudes if you did not slow down.


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Material alternatives for aircraft leading edge
Are there any material alternatives for the aluminium used on the aircraft leading edge? By alternatives, I mean composite alternatives.

Many smaller aircraft have Deicing Boots made out of rubber on the leading edge. Do you need thermal anti-icing in your design?

Yes thermal anti-icing, also the composite should possess good impact strength. I tried searching the composites but then they had very low melting temperatures. So it was kind of a problem. And electrical anti-icing would just draw a lot of amperage.

You can use many materials, are you asking for some sort of list?

That makes it a lot more difficult for composite materials. The 787 uses aluminium leading edges despite electrical anti-icing.

So are you really asking, "have I missed some composite material with a high melting point?" Ceramic-metal composites exist but I doubt they would be a viable solution. Ultrasonic de-icing has also been tried, but of course it brings associated fatigue problems.

Couldn’t you use TKS weeping to chemically deice without needing high heat or a pneumatic rubber boot?

A TKS might not be for a transport category aircraft, might require a ton of deice fluid, which will add weight.

If we're talking about transport category airplanes, the options depend to some extent on the wing.  Wings with hard leading edges (no slats) and supercritical airfoils with poor stall characteristics are not very tolerant of what is called "runback ice"; ice that melts or is prevented from freezing by a warm but not hot leading edge and can run back and re-freeze just aft of the leading edge, forming a ridge, creating a dangerous risk of premature stall in the approach configuration.
These wings may require "evaporative" anti icing, which heats the leading edge to above the boiling point, so that supercooled drops that strike immediately turn to steam and can't flow back onto the wing plank as water.
Such a leading edge will run at about 110 degC when anti-ice is on and you will burn yourself pretty nicely if you touch one.  The leading edges can be aluminum since the aluminum's heat treat state isn't affected until over 200C. So a composite material would have to have a transition temperature above 200C to have some margin to be tolerant of overheat cases.
If your wing has slats, runback icing isn't a concern in the approach configuration with slats extended, so the slats themselves may only need to be heated to about 50-60 degC, and the composite options become wider.
If you're not finding any high temperature composites on the web, try searching just for high temperature Epoxy laminating resins.  There are all sorts of resins that can handle temperatures almost as high as aluminum and some that are higher.
In any case, the biggest reason composites aren't used for heat-anti-iced leading edges is poor conductivity. If you heat the leading edge with bleed air blown inside (typically) or with electrical elements bonded on the back of the skin (I think that's how Boeing does in in the 787), you need the heat to transfer to the outer surface immediately with almost no temperature loss (so that a sensor mounted on the inside surface is reading very close to the actual temperature on the outer surface) and you generally want a smooth surface, so aluminum is the ideal material.
Of course you could go with rubber boots and make the leading edge out of whatever you want, if you're willing to live with the inferior performance at ice shedding. Airplanes typically use boots because their engines don't make enough surplus bleed to heat a leading edge. Just about all turboprops and some small corporate jets like the earlier Citations are like that - and some are able to heat the leading edges but have to use boots on the tail, or, make the tail big enough to handle the ice so you don't have to anti-ice it at all (the CRJs do this).

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

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How does the B777 Side brace locking mechanism work?
The B767 has jury strut actuators and their working is pretty straight forward. They oppose the collapsible geometry of jury strut and lock them in the over centre locking position.

But for B777 this doesn't seem to be the case, I sure can see a small locking piston, but I quite don't understand its working, it seems different from the 767s simple strut locking mechanism

Would be great if you could provide explanation with some picture references.

The scissors struts on that gear physically lock in alignment instead of over centering.  The inboard strut extends into the outer one a short distance, which is forked to accommodate it.  The little actuator with the lever and springs is the downlock mechanism.  The downlock will have some sort of pawl or plunger interface hidden inside that forked interface, roughly where the downlock safety pin is, that engages to lock the inner strut and outer strut together so they are like a one-piece strut.
The actuator pulls in to disengage the mechanical lock, and the springs provide a counter pressure against the actuator to provide a passive "engage" force the rest of the time.  The springs aren't maintaining an over-center condition, only applying pressure to a mechanical lock.
In this video you can see that the downlock release actuator appears to also pull the links out of alignment once the release lever it operates is fully released, and if you watch full screen you can make out the locking mechanism as the struts fold, although the precise details of that mechanism would require access to a 777 maintenance manual.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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What crosstrack should I have during a holding pattern?
I want to know if the ATC is expecting a maximum crosstrack depending of the plane's speed.
I've learn to be at 2.2nm for my holding (with 175kias) but is it just a distance to help me or a rules ?
I think that my protecting side is higher if my speed is higher too but can I use the whole protexting side  ?
Thx

If you follow the holding guidelines you will never leave the protected area. This includes performing an appropriate entry, applying correct wind correction, and flying the correct leg length.
ATC expects you to slow to holding speed no later than 3 mins before arriving at the holding fix for planning purposes, and generally doesn't expect you to use the entire protected area. However, the entire area is protected, so you could use it if you wish.


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What is the difference between a ground controller and an air traffic controller?
In terms of their roles at an airport. I'm designing a system that would inform its users if there is space on the runway that a pilot could use to land or take off, when there is an obstacle on the runway. From my understanding, a ground controller would use this system however the roles of a ground controller and air traffic controller seem to be similar to me so I'm unsure if both would use the system or just the ground controller.

Which is exactly why I asked this question. I'm still in the "research stage" and I started off with pretty much no knowledge. I haven't started any of the design process yet.

Oh, OK. "I am designing" sounded like you were already working on it, rather than researching before starting.

Ground controller is a "position" (role) carried out by an air traffic controller.  The role of an air traffic controller who is working the ground position is to coordinate traffic moving on taxiways and to and from runways.  The ground controller is in charge of the taxiways.  An air traffic controller may be certified to work multiple positions.
You can think of a ground controller as a type of air traffic controller.
Positions (roles)
There are many air traffic controller roles which an air traffic controller may perform.  Other air traffic controller positions are:
Clearance delivery: A clearance delivery controller gives the pilots their clearances, which details their route.
Ground controller: A ground controller is responsible for traffic moving on the ground along the taxiways.
Tower controller.  A tower controller is responsible for traffic landing and taking off or in the air in the immediate vicinity of the airport.  The tower controller is in charge of the runways.
Departure controller.  A departure controller controls traffic which is leaving the airport.
Enroute or "center" controller.  An enroute controller handles traffic which is flying between airports.
Approach controller.  An approach controller controls traffic which is approaching to land at the airport.
Some air traffic controllers work just one position.  Others work multiple positions.  For example, at a small airport, the same controller may be the ground controller and tower controller.  Or an airport that is busy enough in the daytime to need separate controllers for ground and tower may be need only one controller for both roles at night when there is less traffic.
Example flight
Let's take for our example a pilot who is departing from a large airport, flying under instrument rules, and land at a small airport.
The pilot might first contact clearance delivery.  Clearance delivery will give the pilot her route, which she will write down.  The pilot will then radio ground and ask for permission to taxi.  The ground controller will give the pilot permission to taxi, including the specific route along the taxiways to use to get to the departure runway.  On reaching the runway, the ground controller instructs the pilot to contact the tower controller.  The tower controller will then clear the pilot to take off.  The pilot enters the runway and takes off.  The tower controller instructs the pilot to contact the departure controller.  The departure controller gives the pilot instructions to control the route and altitude during departure.  The departure controller then instructs the pilot to contact an enroute controller.  As the plane moves along, the enroute controller instructs the pilot to contact a different enroute controller.  That controller gives the pilot instructions to descend and approach the destination.  The enroute controller instructs the pilot to contact the tower.  The tower gives the pilot instructions to approach the runway and land.  Since this is a very small airport, the tower and ground positions are handled by the same controller, and on the same frequency, so after the plane touches down, the same controller gives the pilot instructions to taxi to her destination.
Every person the pilot talked to was an air traffic controller.  Each of these air traffic controllers was working different positions (roles).
See also: Air Traffic Controller (wikipedia)

Thank you so much - this really cleared it up for me. Is there something in common that all of the different types of air traffic controllers share? (So how would you generalize what an air traffic controller does with a high-level description?)

@xay57n Welcome to ASE.  I'm glad this helped!  There is an up-pointing triangle you may click on to vote for this answer--that's how (and others) reward a good answer.  In addition, you should wait a few days for other answers to come in, and then you can select the checkmark next to the answer you think is best.  This helps other people with the same question as you to see which answer they should read first.

@xay57n I'm not an air traffic controller, but I think it works like this: Air traffic controllers in the U.S. all share the same basic training that prepares them for air traffic control.  But then there is specialized study and training for each position.  For example, a controller fresh out of school may know the basics of tower, ground, approach, etc., but will have additional study'/training before working LAX approach or DFW tower.


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What is the weight of the fuselage (only) of the Boeing 747?
I'm wondering about the weight of the Boeing 747's fuselage.

The fuselage weight for 3/4 of a 747-300 Combi minus engines, fuel tanks, seats, catering, cargo handling, navigation gear, control surfaces, pumps, hose is 70,000 lbs, according to Big Imagination, the people that bought a 747, gutted it and took it to Burning Man. The 70,000 lb figure was used by the Nevada DOT to determine the oversized vehicle permit fee. If you add another 25%, you come up with roughly 93,000 lbs.
In comparison, a A 747-300 dry weight with engines, seats in airworthy condition is about 384,000 lbs.
I know this only partially answers your question, but every other source I have tried features the Operating Empty Weight (OEW), which is a basic working weight for each plane variant.
Additional Resources:

https://www.bizjournals.com/seattle/blog/techflash/2015/02/boeing-747-to-become-largest-ever-burning-man.html
http://www.bigimagination.org/747
https://en.wikipedia.org/wiki/Boeing_747

Good luck finding a better figure. I'd love to know myself.

From Torenbeek, Synthesis of Subsonic Airplane Design, Table 8-5. Weight of the fuselage group of the 747-100 and 200 is about 72,000 lb = about 10% of MTOW.

excellent find!


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Are Dreamliner windscreens periodically replaced irrespective of damage?
Does Boeing have a statute that commands that the windscreens are changed for new irrespective of no signs of damage or fatigue, and if so, what are the intervals, for safety reasons?

First, Boeing does not provide any statues for when maintenance is performed. They provide guidelines and recommendations through the Maintenance Planning Data (MPD) manual that most airlines follow. Some of these recommendations are required to be performed by the FAA with the certification of the aircraft or later AD action.
That said, there are no recommended maintenance tasks for the 787 windshields. The reasons is windshields are certified to be able to to hold cabin pressurization even with every pane shattered. Although there is no recommended or required task to replace the windshields at a set interval, some airlines may choose to do so for operational reasons.
Mandatory part replacements based on flight hours or cycles is almost exclusively reserved for a special classification of hardware known as "Safe Life Parts" these are generally just the Landing Gear and Engine Rotors. These are required to be scraped at specific intervals due their failure mechanisms and because they can be single point catastrophic failures.
I have specifically reviewed whether or not hard time replacement of 737/757/767 and 777 windshields made sense at a very large airline. The data showed that windshield failure rates when plotted against either cycles or hours had very large standard deviations. This means it is very hard to set an appropriate hard time interval, if you set it too early you would have few failures in the field but you replaced a lot of windows that had a ton of life left on them, if you set it too late you still had many field failures and were still throwing away good windows. When doing a Wiebull analysis I was able to identify at least 8 different failure mechanisms that all had very different rates and only affected a small population of windshields. To highlight the point, we had multiple windshields that failed after less than 100 flight hours while we had many that lasted more than 60,000 flight hours (and a few that were up to 80,000 that had never been replaced).

Sometimes a person with exactly the right knowledge comes along to answer a question. Thanks!

Safe Life applies to all kinds of Line Replaceable Units, generally those with a structural element in their use, like flap actuators.  The life limit is usually on the root structural item that defines the LRU's serial number (like a housing), and set based on threshold crack formation during cert fatigue testing. Windshields don't go through that kind of fatigue test process and lamination failures are considered random. The result is they are maintained on condition. Units that don't crack crap out from loss of optical quality from erosion of outer glass or failures of the heating system.

@JohnK that is true about windshields, but at least on Boeing Aircraft, the only Safe Life parts tend to be landing gear and engine rotors with very few exceptions. The 787 currently has some systems safe life parts, but Boeing is working to remove them with further testing/re-design.

Thank you for your interesting & informative answer.  As a consequence and being specific, are you saying that the Mexico - UK flight could have safely continued it's journey, with just one outer cracked yet intact windscreen, instead of aborting after approx an hour and returning to base, which resulted in a 36 hour delay ?

Delay was due to a substitute plane being dispatched from the UK along with a replacement screen and engineers. If that is correct, does it not follow that the operator cannot escape liability for delay compensation under Regulations in Europe EC 261 on the grounds that the delay was beyond their control, given the delay was caused by the deliberate decision of the pilot aborting which now appears to not have been necessary ?

@PeterLynch There are many things that are technically safe to continue the flight with, but will almost always result in an abort. Aborting for a shattered windshield is pretty normal. Any time you have a failure that causes you to go to a redundant system, you have lost some margin making you less tolerant of the next error. In the case of the windshield, if the pilot's windshield fails, the co-pilot must land, and now if his fails you might have to use auto-land, but auto-land can be on MEL. So if you are an hour into a 12 hour flight, turning around can be the best option.

Thank you for your information


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Anyone able to help identify what aerofoil this is?
Firstly, apologies for the potato quality of the image below. I am attempting to determine the qualities of a VTOL and cannot find any data online. If anyone knows what aerofoil this could be that would help a bunch! (the green line is the upper edge and the yellow line is the lower edge)

Do you have any other info regarding where this is from? Is it properly scaled? It looks quite thick, as if it was distorted, or a special root-section foil.

The video I got this from is this one: https://www.youtube.com/watch?v=LeFxjRMv5U8 @ 0:14 (I used the step timer to capture this screenshot when the wing was bent to be in line with the camera)
This is the only image I can find of this craft of the aerofoil section. The chord is 880mm (measured from another drawing). Really just need to find the lift and drag polars so therefore need the aerofoil.

It is not even listed in the airfoiltools database. The closest is the Wortmann FX 08-S-176, but the Cora airfoil has its maximum thickness much more forward which should give it less laminar flow but gentle stall characteristics.

Try contacting the local FAA Flight Standards District Office (FSDO-'fizzdoe') for help. They may be able to tell you how to get a copy of the aircraft's certification documents which may indicate the airfoil used. Mention the aircraft's N-number: N301XZ. To locate the nearest Flight Standards District Office, see here:
https://www.faa.gov/about/office_org/field_offices/fsdo/

… or you look it up here: https://registry.faa.gov/aircraftinquiry/NNum_Results.aspx?NNumbertxt=301XZ. It seems to be an electric rotorcraft, manufactured by ZEE Aero and called Mule SPA. That does not help much.


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Robinson R22/44/66 Headset Plug For Bose A20 Headset
What type of headset connector plug is used on R22/44/66 Helicopters?
Is Bose A20 Headset a good one to be used on helicopter?

Helicopters use a U-174/U or U-93A/U single-plug headset connector. Most aviation headsets come with PJ-068 & PJ-055B or equivalent. You will need an adapter. The second part of your question calls for an opinion, which is out of scope for this service.

All helicopters, even those of GA?

You need an electret mic with an R22 headset IIRC @IvanObodovsky, you'd need to check to make it has one, other than that I agree it's opinion based.

Yes, all including GA. That's the standard for helos.


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What are the requirements for aircraft marker cones?
What are the requirements on aircraft marker cones (a.k.a. safety cones) according to IATA IGOM-96, i.e. size & weight?

For those that are voting to close this question as "unclear what you're asking": The question is about the safety cones that are places around an aircraft to mark the wingtips etc. They need to have a certain size (to be visible) and a certain weight (not to be blown over the manoeuvring area in the slightest of wind), and there are possibly more requirements. The question asks about these requirements.

According to IATA Airport Handling Manual, safety cones should

be conical in shape
be of a minimum height of 750 mm (22.2″)
have a minimum base weight of 4.53 kg (10 lbs)
be orange in color with reflective striping.

I write (non aviation) indistry specs for work . "Be conical in shape" is a spec speak equivalent of "don't be cheeky mate, you know one when you see one"

Thanks so much!!!!!!!

This is what I was searching for!!!


text:
What are the FAA/CS requirements for rubber chocks?
What are the FAA/CS requirements for rubber chocks? Do they include material, manufacturing processes, etc.?

I have never seen any FAA requirements on chocks in my reading of the Federal Aviation Regulations (FAR) or the Airmen Information Manual (AIM).  I used to use some pieces of triangular shaped wood in front & in back of the main landing gear wheels.


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What does unmixed mean?
For a unmixed turbofan engine what does ‘unmixed’ mean in regards to the flow of air?  How does this differ from ‘mixed’ also in terms of air flow ?

I believe this refers to the cowling design where a mixed flow the fan and gas core streams mix inside the cowling whereas in unmixed streams the fan air excites the cowling without mixing with the gas core exhaust gases.


text:
What's the difference between ACARS and ATSU?
I've seen ACARS and ATSU used interchangeably and I've seen references that imply they're different things (ATSU being Airbus only).
How would I explain the difference to someone?

ATSU is a computer. It is the successor to the ACARS Management Unit (MU). ATSU is at the center of the FANS development of Airbus aircraft.
ACARS, as a function, provides Aircraft-Airline communications using text and reports (AOC function). CPDLC function, on the other hand provides Aircraft-ATC communication using text (ATC function).
ACARS, as a datalink, provides communication infrastructure for both AOC and ATC functions.
In the future, the ACARS communication network will be superseded by something called Aeronautical Telecommunication Network (ATN). It will be like the Internet where other networks can get hooked up.

Welcome to Av.SE! Nice answer - please come back often.

While technically the above statements are correct to some degree ATSU can also refer to Air Traffic Service Unit and as defined in Skybrary (https://skybrary.aero/articles/air-traffic-service-ats) this could also be a reference to a provider of aviation services such as air traffic control. The ATSU that @MANN is referring to is an avionics computer that is built by Airbus but that is just a name (like Kleenex tissues).

Aircraft Communications, Addressing and Reporting System (ACARS) is technically any equipment that meets the specification by which aircraft communicate with ground systems including but not limited to CPDLC (Controller Pilot Data Link Communications) a sort of structured text based command communication specification, ADS-C (Automatic Dependent Surveillance Contract) specification as well as AOC (Airlines Operation Center) type communications which could range from messaging for services such as refueling or deicing to proprietary communications between an airline fleet and its owners


text:
Why was there no issue with the Tu-144 flying above land?
The Concorde didn't fly supersonically above land, it only could above the ocean. The Tu-144 however flew between Moscow and Almaty, so entirely above land; why did it fly like this? Didn't it have a that loud sonic boom? If no, why not? If it had, why did it fly above land anyway while the Concorde refused to go supersonic above land?

In the soviet union nobody was going to complain...

@GdD Why would anyone complain about a not frequent sound that is very high in the atmosphere (the Concorde flew up to 60,000 ft and the Tu-144 to 66,000 ft) anyway unless the planes fly at night? Folks beneath the Tu's flight path obviously managed the flights above them well, didn't they?

I don't understand the downvotes. What's wrong with my question? It asks on whether there may have been a technical advantage over the Concorde that allowed it to fly above land. Tu-144s seemed to be more progressed anyway.

I don't know for sure @Giovanni, I didn't downvote but I didn't upvote either. My constructive feedback is that your question needs a bit more research. Did the Tu-144 fly supersonic on it's regular route? Did the route take it over major population centers or were they avoided? You need to account for the nature of the soviet union as well, people were hardly in a position to complain.

@Giovanni Your question assumes a technical difference, but it was actually a political/legal difference.

@Giovanni: I suspect the downvotes, like mine, are because the answer is so blindingly obvious, and belongs on the Politics site (or perhaps History).  To repeat what others have said, this was the Soviet Union.

The Concorde did fly over land on occasion. It was very rare. The route was carefully planned. And, it only flew over sparsely populated terrain.

Comments are not for extended discussion; this conversation has been moved to chat.

other important point: Concorde was operated during several decades, while the Tu-144 did not really get its way to any real commercial service.

Folks, note that any comments are subject to DELETION.  A block of comments has already been moved to chat (to preserve them and continue conversation) and that can't be done twice.  Mods' only cleanup option now is deletion.

Nowhere does your question state that you are asking about technical reasons as your comment claims. Therefore, most readers will assume you are looking for any reason, and the most obvious reason is "because USSR", which is blindingly obvious - hence the downvotes. If you wanted your question to be about technical reasons, you should have included that caveat in the title and body of the question. Don't complain when you weren't specific enough to prevent others from making assumptions.

Answers to this question and comments are so rusophobic and racist, that I'm simply flabbergasted and left without words. The level of anti-Russian propaganda evidently never subsided in the West that you all think that people in Russia were deprived of basic rights even the right to live in quiet places. Nevermind, that during Soviet times airports were much further from the cities exactly for this reason. Oh, the hatred towards Russia in the West... I have no words to describe it.

@Eugene Morozov: Racist?  Hardly: even the most diehard KKKers consider Russians to be anything but white.  Anti-Russian?  No.  Anti-Soviet, certainly.  It's the same as the difference between being anti-Nazi and anti-German.  And people in the USSR certainly were deprived of most basic rights: learn a bit of history.

@jamesqf I was born in USSR and lived in USSR until 1991 when it was destroyed. People in USSR had more rights than in most other countries. It's an arrogant and wrong misconception that Soviet industry would make a noisy plane and make it fly over people heads just because people comfort doesn't matter. It's so wrong, to the point of being stupid.

@Eugene Morozov: You must have a very strange definition of rights.

@jamesqf What can you know about rights of soviet citizens if all you know about it is antisoviet and antrussian propaganda? Just reading the answers here makes me want to do a facepalm - it's not like in USSR the government was obsessed with making life of people worse. On the contrary, it was the first (and the last) state on Earth dedicated to making life of all its citizens better. Not some groups of citizens - all of them. So there were strict norms what is allowed and what not, regarding pollution, public health, labor legislation, etc.

@Eugene Morozov: This is not the place for a political discussion.  I would just suggest that you are probably just as delusional, in your own way, as the typical American Trump supporter.

@IanKemp Read the third and fourth sentence in my question summary again!

The Tupolev Tu-144 was just as loud as the Concorde. As it was already pointed out, the Concorde was legally prevented from going supersonic over land by the US, UK, but it was more than capable of going supersonic over land. There were no similar restrictions over the Soviet Union for the Tu-144. Both planes had a sonic boom.

The plane's chief designer, Alexei Tupolev, whose father Andrei designed the first Soviet jet, was aboard and acknowledged the noise problem, saying it was inevitable with supersonic flight.

"The sonic boom is no different that a thunderclap - so it is no different than nature itself," he said.

https://www.washingtonpost.com/archive/politics/1977/11/02/soviets-launch-sst-flights-with-cognac-caviar/82ab3b61-bf12-401a-8eb1-eda9f6d9be72/

From what I can determine, the noise from the Kuznetsov NK-144A turbofan engines that powered the Tupolev Tu-144 were not quiet and comparable to the noise of the Concorde with Rolls-Royce/Snecma Olympus 593 engines at takeoff produced 119.5 decibels. There is no side-by-side comparisons of the noise.
The Tu-144 was much louder inside during flight because of the amount of air it needed to move to keep the skin cool. The Concorde used more advanced cooling methods to keep the interior noise down.
One additional point is that each of the four NK-144A engines produced 6,000 lbs of thrust more than the Olympus 593. Even though the Tu-144 weighed 22 tons more than Concorde, it never really flew at full capacity, so it had a substantial kick during takeoff.
Now if I could just go on a flight with either plane.

The Tu-144 was originally louder also because the "first gen" engines were not powerfull enough for supercruise, so afterburner was needed to maintain supersonic speeds. Later engine upgrades made it possible to switch afterburner off during cruise, but this didn't affect the sonic boom, of course. It sayed the same.

The sonic boom is generated by the front of the plane pushing air faster than it can move away. It will generate one even if the engines made no noise.

plus a million for that last sentence m'man - shed a tear for the lost days of speed

@Fattie: What speed?  The Concorde & Tu-144 don't go any faster, perceptually, than a subsonic commercial airliner.  If you want fast in an airplane, take your Piper Cub out and fly with the wheels in the sagebrush.

@jamesqf: I perceive 3 hours London to New York as fast (Concorde). I perceive the same journey by 787 in 6 hours as not so fast. I consider a Piper Cub slow in any configuration - unless it was tied to the back of a Concorde!

@user33214: Look out the window, and tell me the Concorde is going fast, and the Cub not :-)  Besides, the time spent in the plane is only a fraction of the trans-Atlantic trip.  You spend way more time on the ground than you do flying.

@jamesqf: When I'm heading west and the sun is rising in front of me, then that's a very good perception of flying faster.

The actual 'legal' reasons have already been mentioned. However, there was a bit more to it.

Tu-144 was meant to fly over land from the beginning; there was no way around it, unlike Concorde. So it was designed to fly higher. In particular, Tu-144 had about 20% lower wing loading and 20% higher thrust-to-weight ratio (at MTOW) than Concorde. (The reality was a bit more complicated; for example, Concorde was limited to FL600 again for 'legal' reasons, due to certification of the pressurisation system AFAIK).
Soviet citizens, apart from not being asked their opinion, were quite used to sonic booms from military aircraft. In many places of the country, they wouldn't even raise an eyebrow. In the Urals where I grew up, I heard them almost daily.

Concorde was also meant to fly over land. Many of the intended routes were between Europe and Asia and cross-continent in the USA.

Your answer, with the personal experience, is the most useful IMHO.

Regarding the last point: This is certainly true for other Eastern bloc countries too. In the GDR I heard supersonic booms every Wednesday, when an Soviet airfield nearby had their flight training day.

When I grew up in the 1980s sonic booms were quite usual in the northern FRG, too. I am too young to actually remember, but in my memory it was a daily thing. The disregard for civilians on the ground was not a Eastern Bloc thing, it was a cold war thing, or an 80s thing...

@I'mwithMonica, As an American who visited FRG in the late 80's, I was shocked by the number of sonic booms I heard every day. I never heard one at home, despite living within 50 miles of 2 major USAF bases. It was a Cold War thing, but the burden wasn't shared equally by all participants in the Cold War.

The Concorde didn’t “refuse” to go supersonic over land; it was legally prohibited from doing so by every country it flew to/over.
The Tu-144 produced the same sonic boom, but aside from a few exhibitions, it flew only to/over countries that had no such law.

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Another reason is that much of the flight was over the huge land mass of Kazakhstan with very low population density. So few people live in the steppes there that Roscosmos lets the first stage of rockets taking off of Baikonur simply fall to the ground. You wouldn't want to do that over France or England.1
Population density is higher in Russia close to Moscow, but still lower than in Western Europe, so with a suitable flight corridor most of the flight would be over sparsely populated areas.

1 Bear with me when I divert a little. For details about the first stages falling from the sky in Kazakhstan I recommend the stunning documentary Space Tourists by Christian Frei. One of its threads follows Kazakh metal scrap collectors who hunt them down. You will be surprised what they do with the round bottom part of the fuel tanks on the first evening after finding a stage.

"You will be surprised what they do with the round bottom part of the fuel tanks on the first evening after finding a stage." This is not a news site trying to get clicks.

@Sam I am not involved in any way with that movie, if that's what you are implying. Or did you mean I sound like click bait? I just didn't want to give it away. The movie is worth buying or watching if it's still available at all. And while the movie and this detail are off topic on this site, they aren't by much (hey, the first stage is flying, in the atmosphere!). But that's why I put it in a footnote and said "bear with me".

@Sam By the way, do you think this answer has any merit concerning the subject matter? I think it is one main reason -- if Kazakhstan were as densely populated as England or France then even in a Soviet totalitarian regime the plane would not have flown that route (but instead to Irkutsk or some other Siberian place).

@Peter-ReinstateMonica, it's an important distinction that the USSR had lots of sparsely populated land to fly over without bothering too many people.

In addition it's also worth noting that the TU-144 only made 102 commercial flights. It wouldn't really have got to the point where anyone would complain.

Welcome to Av.SE!  Can you provide a link to that statistic?  With so few flights, no wonder nobody complained!

@RalphJ, Wikipedia quotes the BBC about that.

But the Tu-144 was used by the military until 1999.

@Giovanni, western military jets flew supersonic over land regularly, as mentioned in comments to Zeus's answer.


text:
How high can manned subsonic planes go?
What is the maximum altitude a manned plane that flies subsonically can reach? Subsonic airplanes can't go as high as super- and hypersonic ones of course, so record holders are the latter ones. But ironically, the altitude record for leveled flight is set by the subsonic NASA Helios propeller aircraft at ~97,000 ft. However that's an uncrewed and weird-looking one. Can manned airplanes that fly subsonically go above the Armstrong line (60-63,000 ft) and higher (either in leveled or in parabolic flight), considering realistic fuel limitations? How high can subsonic airplanes go compared to supersonic ones?

76,000 feet.  Airbus Perlan Mission II glider, Sept 2, 2018.  (If Airbus keeps changing their press releases' URLs, see also Wikipedia.)
The U-2 has a higher service ceiling, 80,000 feet, but it hasn't reached that.
(Perlan II's service ceiling is 90,000 feet.)

But that's a glider, how did it get into the air in the first place? Do the Perlan gliders launch on winches?

@Giovanni a glider is a plane, and it is both manned and subsonic, thus fulfilling all your criterias set in the question, no?

Look up the U2.

@Federico Not if another plane brought it to or close to that altitude. I'd count it only if the glider was released at low altitude or launched on a winch.

@Giovanni but you did not specify as much in the question

@Federico If another plane brought it up there, I'd count the other plane's flight (if it's subsonic). But you're right in that I rather meant jet planes. Still, interesting. Perhaps I'll edit my question sometime to set the focus on jets.

@Giovanni: If it was brought to that altitude by another plane, then that plane could obviously fly to that altitude.  But it wasn't: it's a sailplane: it's released at low altitude and uses wave lift to climb.  Likewise, you could put someone in a pressure suit aboard a Helios-like craft.  And then there's NASA's Mars helicopter: https://mars.nasa.gov/technology/helicopter/

@jamesqf Thank you. I should have specified I rather meant subsonic jets, not gliders or propeller craft since these use in part other kinds of buoyancy. But I wonder whether a manned Helios-version wouldn't be too heavy for such high flight.

@jamesqf My question is about fixed-wing aircraft, not pure rotorcraft.

According to Wikipedia the U-2 has a service ceiling of 80,000 feet.

@MichaelHall I see, the U2 is subsonic. Thank you.

@EricS Yeah, also found that out just now by googling it ('ve been too busy before).

@Giovanni: Gliders & propellor airplanes do not use buoyancy.  Balloons use buoyancy. Per Wikipedia, the record for an unmanned balloon is 173K ft, manned a bit over 135K.  WRT the Mars helicopter, the point is that if a rotorcraft can work at such low atmospheric pressure, then prop planes should.  And WRT jets, I think at very high altitude you'd need either a rocket or an electric motor - but I defer to anyone with more knowledge.

@jamesqf Gliders do use buoyancy after they're released or what do you call it? I'd call gliding up a kind of buoyancy. But what I was referring to is something that keeps them from falling down (airflow).

@Giovanni: Buoyance is when an object weighs less than the material it's immersed in.  Balloons weigh less than the surrounding air, so they go up until they reach equilibrium. (Likewise ships displace an amount of water equal to their weight.)  Gliders climb by means of lift: they get into a bit of air that's rising, and climb along with it.  The lift can be from wind blowing up a slope (as with hang gliders on the coast), from thermals (most common with sailplanes), or mountain waves, which (as with the Perlan) can take you really high when conditions are right.

@Giovanni: To be clear, a glider is always going down relative to the surrounding air (ignoring maneuvers where you trade off speed & altitude: that's a net zero).  It's just that you can find masses of air that go up faster than the glider goes down.

It looks like the link is broken


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Could the SR-71 Blackbird be used for nearspace tourism?
The Blackbird had two seats onboard. Could it be reactivated for nearspace tourism, placing a paying tourist in the other seat? If no, why not or why isn't it being proposed? In leveled flight, the Blackbird flew up to 90,000 ft (27 km) MSL and faster than Mach 3. In a ballistic parabola it could probably reach an even much higher altitude, and on the parabola's top you'd start to experience weightlessness, go above most of the ozone layer, see a black sky with some stars in daytime and the thin blue atmosphere of Earth. An exceptional experience for many.

Comments are not for extended discussion; this conversation has been moved to chat.

Significant issues with the concept: the SR-71 lacks RCS, so you'd lose attitude control if you got significantly above normal operating height.  I originally wrote that it doesn't handle in-flight restarts well, but apparently it has the opposite problem -- it needs them all too frequently in normal operation.  It's also not an aircraft built for near-vertical climbs or high-G maneuvers, so a zoom climb is outside its normal operational envelope.
So, you can't demonstrate long (> 1 minute) periods of microgravity without the likelihood of loss of aircraft, crew, and paying passenger.  Beyond that, however, there's virtually zero visibility from the rear seat.  Who'd pay $100,000 for a "near space experience" with no zero G and no view of the black sky and curved horizon?
Never mind the fact that the SR-71 doesn't run on standard jet fuel (JP-1 or Jet-A); the engines could be retuned to do so, but then wouldn't continue to operate at 80,000 feet ASL or push the aircraft to around Mach 3 at that height -- in other words, it wouldn't do what an SR-71 is famous for doing.  And the special fuel the SR-71 requires hasn't been manufactured since the aircraft were retired -- it would probably cost more to restart production of that fuel than you'd make back in a reasonable twenty-year "space tourism" operating life (never mind cost of refurbishing an aircraft, pulling still-classified equipment out of at least the passenger seat, training new pilots -- oh, right, you'd have to refurb one of the trainers, that would at least solve the passenger visibility issue)...
Bottom line: no chance.  You'll get a better experience with the Russians; at least they can do a zoom climb and have RCS to point the nose back down so they don't wind up in a flat spin like the one Yeager had back in the day.

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I work and have worked with several SR-71 crew members.
In the past, they have pointed out two principle impediments:

Crew workload
Cost

Crew workload is shared between both crewmembers, and the workflow would need substantial modification and equipment changes to permit the backseater to merely be a passive observer.  Not to mention that the backseater really doesn't get much of a view.  So aside from crew duties, there would be little experience which would warrant...
The cost, is high for even SR-71 training missions.  A friend who flew as the backseater in the NASA SR-71 commented that their cost per flight was 5 to 6 million.  Tankers were necessary, and the equipment was not the most reliable.  Once when taking their SR-71 to Oshkosh, they were looking at corn tassels while working out a fuel transfer pump problem.  The aircraft sat at Milwaukee for a couple of months while repairs were effected so it could be flown home.
Not to mention other endearing attributes of the SR-71.  Frequent fliers complain about engine unstarts and the violent head jarring experiences they bring.  It is just not economically feasible and lacks a good customer experience.  Better options for near-space are available in Russia.

Or in Virgin Galactic (SpaceShipTwo) and Blue Origin (New Shepard). There will also be a SpaceX Dragon orbital tourist flight.

Absolutely, however I did not reference those because the Russian option is here today.  I think that there is a similar existing option in one of the Eastern European countries as well.

I think many countries offer flights in fighter jets (including weightless parabolae), the question is just how high they (can) go. As for the Russian one, they state they're the only option for a civilian to break the sound barrier since the end of the Concorde, so in this they're exceptional. I wonder whether you can go above the Armstrong line subsonically in a plane. I should ask this as another question.

VIP flights were done in the SR-71; see e.g. http://www.sr71.us/Supp_BBook.htm  For the view, perhaps the SR-71B would be the appropriate variant to re-activate if you were going to all that trouble anyway.

To your second point of cost - I was in TAC when there was the proposal to retire them the first time. Remember, the Soviet Union was collapsing and Congress was looking for a "peace dividend" so there were big budget cuts coming at the time. I asked a friend who was involved why they were considering retiring the SR-71. His response was that that one SR-71 wing (~20 a/c) cost as much to operate as three F-16 wings (72 a/c each), though that cost included the KC-135Q tanker support. I was surprised when they brought them back when they did.

Typical SR-71 missions proceeded as follows:

Fill tanks on ground.
Tanks leaking fuel like crazy, by design.
Plane takes off, with fuel leaking everywhere.
Plane gets to altitude, and flies very fast to heat up.
One plane is heated, fuel tanks are sealed, but nearly empty.  Mid-air refueling required.
Plane has to fly as slow as possible / near stall, refueling plane has to fly as fast as possible to refuel.
Once refueled, SR-71 can fly mission.

The above does not include, the extremely high probability of failure for this aircraft, and the very high level of danger.
Aside from all the other logistical issues described by others, it's just not a practical aircraft for anything but top secret missions where the high probability of failure is acceptable, and the crazy amounts of danger are concealable.  As a commercial aircraft it would never work.  It just has way too many failure modes.

"On a typical SR-71 mission, the airplane took off with only a partial fuel load to reduce stress on the brakes and tires during takeoff and also ensure it could successfully take off should one engine fail. As a result, the SR-71s were typically refueled immediately after takeoff. This has led to the misconception that the plane required immediate refueling after takeoff due to leaking fuel tanks. However, leaks were measured in drips per minute, and were not significant compared to the overall capacity." - Wikipedia

That's interesting.  I read first hand accounts 20 years ago, that indicated the leaks were way worse that "drips per minute."

Where is Mary Shafer when we need her? - Example


text:
What exactly do "attempt" and "cycle" mean in these starter duty limitations?
Can someone please explain me this A320 Starter Duty Limitation in simple terms?
E.g. for the CFM56 Starter:

No running engagement when N2>20%
4 cycles of max 2 mins
Pause between
start attempts . . . . . . . . . . . . . . . . . .. . . . . . . . . 20
sec
Cooling period after 4 attempts . . . . . . . . . . . . . . . . .
. . . . . . . . 15min

Does it mean the moment I put Engine Master to ON till Automatic abort by FADEC in case of Automatic Start or Manual abort by turning the engine master and man start button off (limiting it to 2 mins) in case of Manual start = 1 Cycle. Then 20 Seconds gap. Then Next Cycle and so on and then say after 4 unsuccessful cycles I got to wait for 15 min.
But things get confusing between words Cycle and attempt. Since for another type of Engine it says

Auto start with up to 3 start attempts is 1 cycle.
Between
successive cycles – 20 seconds pause (for auto or manual ground
starts).
After 4 failed cycles – 15 min cooling period.

What is attempt and what is cycle? I hope someone explains it in real simple terminology.

Some technical terms have universally agreed upon standard definitions.  Others are ordinary words of the English language whose meaning must be discerned from the context.  The words themselves are just labels, therefore it is less useful to debate their meaning and more important to understand the intent so that you can act appropriately.
To avoid confusion as in the seemingly contradictory examples you show above, I have typically seen starter limitations expressed in table format.  This is how I would choose to depict the example above:

Start #
Max time on
Cool down

1
2 minutes
20 seconds

2
2 minutes
20 seconds

3
2 minutes
20 seconds

4
2 minutes
15 minutes

FWIW, I have heard these terms used interchangeably.  Typically a normal start "cycle" begins when you engage the starter, and ends when RPM is at idle with oil pressure in band, and the starter has disengaged.  If the engine never gets to that point it is a failed start "attempt".  (or failed start cycle, or aborted start, or "try", or...)
I have never heard a series of failed attempts referred to as a cycle, but in the proper context the term would certainly be appropriate.  Since the limitations are there to protect the starter, if RPM never gets above zero I wouldn't even count it as an attempt.  Does this help?

Three start attempts are considered one "CYCLE" (we talk this in terms of starting the engine, that engine will get started in atleast these attempts).
A gap of 20sec or 35 sec after each attempt (failed /successful) is required.
And the cooling period of 15 min after 1 COMPLETE CYCLE (or 4 failed attempts) is required.
hope this helps.

Could you clarify if (i) between 'attempts' the starter remains engaged and motors the engine, and (ii) if the starter has to be reengaged (ie wait for N2<20%) then that constitutes a fresh cycle? Do you find sense in what I'm saying here? (I have no experience on type/engines.)

This isn't how I have heard it defined...  Do you have a source for this interpretation?

Re "Three start attempts are considered one "CYCLE""  and " And the cooling period of 15 min after 1 COMPLETE CYCLE (or 4 failed attempts) is required."  -- This is confusing.  Do you perhaps mean to say that "The cooling period of 15 min after 1 COMPLETE CYCLE (meaning 3 failed attempts) is required."?


text:
Why do we still interpolate in performance tables?
I'm getting my Private Pilot License. In the coursework, we learn about flight planning, which requires looking at performance charts for your particular aircraft. These charts in my case are tables like Cruise Performance or Climb Performance, which, for a given Pressure Altitude and temperature, will give you RPM, Fuel Burn, and TAS.
The tables in my case (Cessna 162) have rows for Pressure Altitude equal to 0, 2000, 4000, etc. So if you're dealing with a Pressure Altitude of 3000 ft, you have to interpolate the values from the 2000 and 4000 rows.
My question is, these values are clearly coming from an equation of some kind that accounts for temperature, Pressure Altitude, maybe some other things. Why not give me, the operator of the plane, the actual equation so that I can plug in the exact values and get the exact answer, rather than having to pick the closest calculated value and then do a bunch of small calculations every time I want to plan a flight?
I'm a student, so I'm happy to be told I'm just supposed to learn it this way for historical sake in case I ever need to fly an old airplane, and that ForeFlight basically does what I want in real life. But I was curious if there was some benefit to keeping that information in tables that I'm just not thinking of.

Why do you believe it's formula's versus actual measurements at those pressure altitudes? Interpolation is quick, do you think you could run the calculation with "N" number of variables while flying the plane?

It sucks but a lot of aviation and aeronautical engineering is indeed measuring with a ruler on black and white graphs made 50 years ago.

@stevederekson555 you can use a flight management app or a on-board system if you have one, it is still good to have the manual ability for emergencies, validating the results and also to have a mental model of such flight parameters

I doubt it's "arrogance", @abelenky, more likely the "ignorance of youth". I'm not a pilot, but I was more than happy to go along with the thought of "plug a few numbers into an equation or two and get the precise answer". No arrogance here, just ignorance. We're all learning and we're all on different places along the journey. If our OP is just staring his journey, he's not much further along than I am, and it's not an unreasonable question.

Re "...in case I ever need to fly an old airplane...", unless you are, or intend to become, quite wealthy, you will almost certainly be flying old airplanes.  You might even if you are wealthy, given the limited production of new planes.

The performance of any reasonably "modern" aircraft (i.e. one designed after say 1970) has indeed been modeled by a "formula" and there are even agreed standards for sharing such "formulas," for example between airframe and engine manufacturers, commercial aircraft operators (a.k.a. airlines) etc. However, the "formula" is most likely expressed as 10,000+ lines of computer code, and therefore no use to avoid the OP's interpolation problem.

Those equations are for the physicist and engineer, not the pilot who has better things to do.

I was in the technical publishing business (flight and maintenance manuals) in another life.  Tables are used as an alternative to graphical plot presentations in flight manuals, and make it a little easier to access information, say, just sitting with the book in your lap.  With the graphical plot, you have to unfold the sheet and have it laid out flat so you can work across the page, maybe needing things like straight edges to help locate data points relative to the margin scales on a graph.  Mathematic formulae are not normally presented because then that would require pilots to do math, and most pilots go catatonic when forced to do math.
Breaking the information into tables shortens the process of sifting the data you need compared to a graphical plot, but in order to save publishing space, the increments of the data presentation are limited to fairly large increments.  A table could be presented that included every unit of information, but to do so might require the table to extend across 20 pages instead of 2.  Limiting the table to, say, 10 unit increments, cuts way back on the space it uses, but it requires you to interpolate, or make an educated guess, on the precise unit within that 10 unit block to use.  When tables are used like that it's because the interpolated "guess" is accurate enough for the job.
The most extreme example of this might be found in Quick Reference Handbooks used by flight crews for procedures and performance data.  The original information for some performance parameter will always be in the Airplane Flight Manual, in the form of a multi-element graphical chart (the root certification document basically), but you normally don't have that handy in the cockpit and if you did you couldn't really use it anyway; you have the QRH.
You are typically using the QRH sitting in the cockpit with a little map light to see, and a graphical plot format just won't work.  The QRH condenses the data down to tables, making it much faster to work out the data you need, and when using the tables, interpolating values between listed increments is accurate enough to do the job.
So it's perfectly fine to see a table that gives a result of 10, 20, 30, and 40 say, and you make a guess that the value you need is between 10 and 20, and 17 "looks about right".  The idea is that if the true value is 16 or 18, it makes little material difference in the real world in that instance.
It reminds me of when the use of electronic flight calculators to replace circular slide rules became popular in the 80s. Everybody I knew was buying those new fangled flight calculators, but I never bothered because I was cheap, and because the super precise result it gave was not something you could achieve in normal flying anyway. The Jeppesen circular slide rule may only be accurate to a knot or degree, whereas the electronic calculator goes down to decimal places, but nobody can actually fly with that precision and the slide rule is good enough if you're not trying to navigate to the Moon.  Plus, if you get good with the Jepps circular slide rule, you can hold it and do solutions with one hand, getting a useable result in way less time than the calculator took with all the key punching.
Using a table in place of a precise formula or a graphical plot is kind of like that.

Next I'm expecting to find out you were once a wing-walker and a fire-jumper. :)

@TomMcW Well I have had a kind of an odd meandering career because whenever a door opened, I always went through it. Lessee... airplane mechanic, rivet smasher, tech writer, real estate agent, bush pilot on floats, tech writer again, tech support engineer regional airliners, little bit of jet time flying part-time production support.  I used to skydive in the late 70s but just for fun.  For sheer fun and adventure, the bush pilot gig was best, but the pay was pathetic.

Also, a common method to simplify using the tables is to just always round up (next higher temperature, altitude, etc.).  If the result is insufficient performance you will need to go back and interpolate to see if it's legal/safe, but it often isn't an issue.

@Lnafziger I'm sure there are cases when rounding to the worse of 2 cases is down rather than up, but anyway this seems like a quick and sensible way to ensure sufficient margin.  If that's all that's required of course.  But such an approach is common across engineering disciplines, as far removed as electronic component datasheets - new ones have pretty graphs, but they're not always as helpful as your method

If a whiz wheel is good enough for Mr. Spock, it should be good enough for anybody!

When tables are used like that it's because the interpolated "guess" is accurate enough for the job. is probably the most important statement. There's no need to be any more accurate, or they'd provide more accurate data (or smaller steps to interpolate between).

@FredLarson, but there's only about a 99.4828527% chance that it's good enough for Mr. Spock...

For most of the information in those charts there is not a single formula which would cover all aspects. The graphical way is the simplest and has other advantages, too:

With a formula it is easy to use a wrong number without noticing. On the graph you see where neighbouring values lead to so you can easily check whether your result is reasonably correct.
Some relations are non-linear, so you would need several formulae, each for its own range of valid input data. The chart simply adjusts the gradients of the curves - in most cases you will not even notice.
Aviation is extremely conservative. If a method has worked before, changing it means an additional risk for little reward. Reading performance values from charts works well enough that the gradual improvement of a formula (if it exists at all!) will not be worth it.

The equations, if they exist, are often rather non-linear and not enjoyable to calculate on the fly.
I do not have an equation for the RPM at a given pressure altitude for a Cessna 162, but if I may use a different equation as a concrete example: pressure correction for given altitude:
$$P(h)=P_0 e^{-\frac{Mg}{RT}h}$$
That equation is the Barometic formula.  It ignores all sorts of real effects, but its a pretty good little equation.$M$, $g$, $R$, and $T$ are constants, so we can reduce it to a more managable $P(h)=P_0 e^{-kh}$.
I don't know about you, but doing exponentials is tricky for me.  I can't do them intuitively in my head, and its hard to confirm that I typed them correctly into the calculator.  But I can do linear interpolation, or check my calculator if I have it do the linear interpolation.
And, frankly, that's one of the easier equations that show up.  The equations you mention speak to the product of very complicated models (or just empirical data that has been sampled).
/Warning - calculus ahead!/
If you are interested, you can actually compute the maximum error caused by this linear interpolation.  Mathematics has equations for this.  Practically speaking, all of the equations you are interested have a 2nd derivative everywhere, so you can use Rolle's Theorem to find the error on this linear interpolation:
$$|R_T| \le\frac{(x_1-x_0)^2}{8} max_{x_0\le x\le x_1}[F^{\prime\prime}(x)]$$
where you are looking from altitude $x_0$ to $x_1$ and $F^{\prime\prime}$ is the 2nd derivative of the function being interpolated or an upper bound on it.  You don't have $F$ (which is the equation you are asking for in the question), but you can estimate it with finite differentiation, such as $F^{\prime\prime}(x_1)\approx \frac{F(x_0) + 2F(x_1) + F(x_2)}{x_1 - x_0}$ and then put a healthy margin on it.
This is a lot of work.  I don't really expect you to do it.  But it does provide a useful comparison.  If one isn't willing to work out a set of error estimates while sitting on the ground, how willing is one to compute the actual equations in the air?
And if you do work out the error estimates, you can now peg them against uncertainties, such as how well can you actually calculate Pressure Altitude given the instruments you have on board?

As has already been pointed out, a lot of the data are not results of a (single) formula. Besides (non-linear) differential equations, even a lot of the algebraic correlations can't be solved analytically. These will need to be solved numerically (like x + ln(x) = 0  ..)
A lot of them are empirical equations themselves, meaning that the equations are not based on physical models, but merely an approximation trying to fit measured values, which is not very different to interpolation. e.g.. Darcy-Friction Factors (https://en.wikipedia.org/wiki/Darcy_friction_factor_formulae)
Some are even both - purely empirical and not analytically solvable:
(for amusement.. scroll down all the way  to "Table of Colebrook equation approximations")
Then there are mixed forms, like correlations derived by dimensional analysis ("pi theorem"), and the coefficients coming from empirical measurents.
Some examples:
Some data are likely the results of design process calculations and a lot of validation measurements:

For fuel consumption at different altitudes and temperatures, I strongly assume that your Cessna 162 has been desiged for certain performance values, and then someone at Cessna has just flown at these conditions and actually measured, and that's the data points you have in the table (plus some data smoothing and adding a bit of margin)

Other data are purely empirical:

which octane number do you need to avoid knock? How does fuel-air mixture ignition depend on octane number?
what is the pressure in the tyre? (for reference - how did you determine the pressure in the tyre of your bike? car?)

Bottom line
Interpolating in a data table is good enough in accuracy (likely not worse than the other methods), but easier  and less error-prone - hence it's done in the cockpit

Additional note: The other methods lend themselves to predictions during the design phase, while the data tables are static and limited to known values within the operating boundaries once the design is fixed.
I wouldn't use those data tables, if you intended to .. change the wingspan?


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Does an rc plane tail also have airfoil or is it just flat material?
I've chosen an airfoil for the wing, now i'm confused weather i should do the same for tail or just attach a flat piece of wood as tail. I'm going for conventional tail design.
This is my first ever plane so sorry for the dumb question.

How big is the plane? How much tail up or down force you need to counter the wing?

@CrossRoads it's a small trainer plane. wing is 1m*0.3m and it has a pitch down momemt of 0.03 at the angle of attack at which it will fly. Moment arm between CG and tail is 1m which means the counter force needed from tail is 0.03N. Correct me if I've calculated it wrong

and yeah it's supposed to be a 1.7kg plane

A flat piece of wood with rounded edges will work fine.
The tail's job of producing downforce doesn't require the lift coefficients or angle of attack range required of the main wing, so the actual airfoil shape is not that important especially at low speeds, even more so under the low Reynolds numbers ("thick" air) that RC airplanes work at.
In fact most fabric light aircraft get by with what is effectively a flat surface wind deflector, like this Super Cub.

No worries.  The more the merrier!

@John K nice picture!  Notice the tail surfaces are flat plate as you say.  They are also low aspect ratio (the vertical is half a delta) because they require a GREATER angle of attack range than the wing.  The wing should stall first.  But it is great that you show these flat plates are fine even on full scale aircraft.

Flat tail surfaces need to be larger than those with a proper airfoil, because the maximum possible lift coefficient is lower. This is not needed to trim the aircraft, but in manoeuvring those limits can count indeed.

The Zenith 701's flat topped cambered tail is probably one of the best examples of that point taken to the extreme, in order to get adequate tail power at the 701's crazy low speed.  The drag of the thing was not such a big deal in an airplane that can only cruise at 80 or so.

@JohnK i get it. But why the rounded edges? how does it help to increase lift?

A little bit more efficient.  AIr doesn't like sharp edges or corners.  Also for looks.  II don't mean planform, just talking about rounding the edges of the flat piece with sandpaper.

Flat plate will be adequate as most r/c planes are overpowered and do not require perfect
efficiency to enjoy.  Much more important are the proportions of wing, Hstab, elevator, and elevator throw.  Finer points can come later.  See what other planes similar to your build have.
If it is your first plane, you may be far better off buying a ready to fly.  Gain some experience.
There are so many variables it is best to start with an easy trainer and hook up with an instructor or somebody with experience. These can be found at local radio control flying
clubs.
In the mean time, you can build simple gliders and experiment with designs.  As in the early
days of flight, the thin undercambered wing will give amazingly slow glides.
Also, check out R/C Groups on the web.

A flat piece of wood is technically not an airfoil due to the lack of curvature, but it may help to think of it as an airfoil with a very, very bad lift:drag ratio.  The tail needs a certain amount of lift to stabilize the plane, so the more drag you incur in getting that lift, the more power you will need to overcome it.  But the tail is small compared to the wing, so its overall effect on efficiency is also small.
Is it better for you to add a little more power every time your plane flies or to add a lot more effort once while building it?  Only you can answer that.

that's right but my plane is supposed to be just 1.7kg, with wing dimensions of 1m*0.3m only. Very little counter force will be needed from the tail. Would i still need to make tail out of an airfoil?


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How do I calculate the tail size of my rc plane?
I'm in the conceptual design phase and my plane is supposed to be of 1.7kg. Wing dimensions are 1m*0.3m and AoA that it will fly at will give a pitch down moment of 0.03. The moment arm between CG and tail is 1m. How do i calculate the size of the tail. It's supposed to be a conventional tail.

What have you tried so far? It looks like you have everything you need there except the $C_l$ of the airfoils and area of the tail.

Haven't done much yet, was still researching on it. Cl of the wing airfoil at the set AoA is 1.4. How do i calculate the area of the tail?
The tail volume ratios and tail volume coefficient seem confusing to me. Do i have to consider these things in my first ever rc plane?

What is your intended speed range? It seems you are setting $\alpha$ and $C_l$, which will give you only one flight condition, and therefore one particular HTP solution, which may be in the middle or on the extreme of your flight envelope. And no, you don't need to consider these things, you can make anything fly, even a folded A4 sheet. But since you are here, I assume you want to consider them.

By plugging in the values of Cl, wing area and lift, I've calculated the stall speed from the lift equation which comes out to be 8m/s. I'll fly it at low speed, maybe not more than 20m/s, since it'll be my first flight. And yes i am setting alpha and Cl from the airfoil graphs, isn't this the way you proceed while designing?

I'm not sure about calculations but the general rule of thumb I know is for small free-flight planes you'd want the h-stab to be around 1/2 the span of the wings. For RC planes of up to 1m wing span you'd want it to be around 1/3 the span of the wings. Larger planes can have stabs as little as 1/4 the span of the wings (indeed, that's what you'll typically see smaller full-scale planes)

There's a formula for calculating minimum tail volume coefficient (tail area X distance from the wing) for light aircraft.  Not sure if the same value applies to RC but it's a good starting point. This is from David B Thurston's book Design for Flying (Thurston was an engineer on numerous Grumman programs going back to the Hellcat).
(Tail Arm ÷ main wing MAC) X (Tail surface area ÷ wing area) = .55 or greater
Tail Arm is the distance from the C of G to the 30% chord point of the horizontal tail.
.55 minimum is for a symmetrical airfoil section tail typical of light planes like Cessna 150s.  I would use a higher number for a flat sheet tail surface, and being RC, so maybe .6 to .8.
While you're at it, Google "tail volume RC aircraft" and you'll find some more information specific to RC that'll be helpful.

Agree with the higher coefficient. RC planes fly really slow compared to real planes (100mph is considered really fast). Combined with the small size of the models and we're dealing with very low Reynolds numbers. I'd err more towards .8 than .6

This is a great opportunity to compare experimental results with mathematical predictions in a scale model aircraft.  Once you have chosen your tail planform, you size it until its torque cancels the forward shift of the wing center of lift during pitch up.  If it is too small, the aircraft will continue pitching up and stall (try no Hstab at all and you'll see)  Naturally, pitching tendency includes pitch components of wing, tail, fuselage, landing gear, nacelles, etc.  The Hstab, as AOA increases, will begin to contribute a nose down force to help keep center of all lifts in the same spot.  This work can be done "the hard way" in flight, or in a wind tunnel (safer). Better to err on the side of large. Your 1 meter pitch torque arm should make this model very stable and easy to fly.  The next step will be area and throw of your elevator for controlled flight.
Comparing tail design of similar models and existing data is a good place to start.
Set your Hstab angle so that when it has 0 degrees AOA (neither up or down force) the wing AOA is optimal (usually for cruise).


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Could the information of flight crew or maintenance personnel be provided to passengers?
Many passengers experience insecure or anxious feelings during flight. If they were provided aircraft performance info, flight crew or maintenance personnel's info(say personnels have many years of experience), which indicates flight is checked and safe, they may not be worried during flight.
I wonder if there's any regulation or law regarding to this issue. Does anybody happen to know it?
Thanks!

...then what would happen to anxious passengers if they were told the crew was relatively young and inexperienced?

The fact that we're going flying, means that I as the captain, along with any number of others in Dispatch, Maintenance, Operations, and other places, all agree that everything is up to standards & we can operate the flight safely.  If that wasn't the case, we wouldn't be pushing back from the gate.  So no, you aren't going to get a massive data dump that includes the coffee maker in the aft galley being replaced last week and 1001 other irrelevant details.

"One compass, and one radio altimeter are broken. An engine is due for maintenance, and the tires will need to be changed after the next landing. Our captain is going through a divorce, the purser is a psychopath, and this plane is 25 years old. According to the regulations, everything is up to standards & we can operate the flight safely. Enjoy your flight and don't worry!"     ;)

If they were provided aircraft performance info, flight crew or maintenance personnel's info(say personnel have many years of experience), which indicates flight is checked and safe, they may not be worried during flight.

Not a bunch of log and personnel files, but various certificates guarantee a flight is "checked and safe". The most important ones I can currently think of are the air operator's certificate (given to an airline) and the airworthiness certificate (given to an aircraft).
To address the regulatory aspects of your question:

personnel info will be subject to privacy regulations and therefore would not be shared with passengers;
maintenance logs are often company confidential, and are therefore not shared.

The FAA makes safety-related maintenance issues public information.

Averaging roughly 300 commercial flights per year, I have yet to meet someone anxious or insecure due to missing aircraft performance info or flight crew info, simply because they would still not be able to gauge relative safety to a self-comforting level. The average person with flight anxiety is essentially irrationally afraid of dying due to little knowledge of publicly available transportation safety statistics and math. Have a look at http://planecrashinfo.com/cause.htm.
While interesting for some, offering information on aircraft performance info, flight crew or maintenance personnel's info - apart from the questionable legal standpoint - with the implicitly needed hinting towards safety would be lying, as there is no quantifiable correlation between such information and safety. I happily stand corrected.

You average 300 flights per year? Realizing that getting from A to B often means stopping in C & D along the way and requires 3 flights, that's still a lot of days in the air! Yikes!

@FreeMan: In the 21hrs between my reply and your comment, I boarded 4 flights in Europe. Yes, it's a lot of days (about 900-1000hrs) in the air. This amount of flying has obviously earned me the highest status with the largest alliances, so I don't have to endure traditional hassle getting from A to B.


text:
Is some sweep beneficial with taper?
I'm developing a canard glider which requires some taper for the canard to stall first. I understand that sweep is typically only for aircraft in a compressible airflow (M >= 0.3), but is there any benefit to having sweep such that the trailing edge is straight?


text:
What is this tube in a jet engine's air intake?

I have long been wondering what is the official name of this little air intake tube located in front of the fan blades and its intended functions. I did search it on google but it was in vain. I guess that it may be a pitot tube installed inside a jet engine for gauging the airspeed of the engine intake air.

It is not a pitot tube because it is open on both ends, and pitot tubes are only open on one end(with the exception of a water drain hole which is several times smaller than the main opening). It is more of a specialized static-pressure port.

That's the EPR inlet pressure sensor (there's another one located aft of the engine in the exhaust).
EPR = Engine Pressure Ratio, one way to measure thrust.

(source)
See also:

What is the difference between EPR and rotor speed as thrust setting parameter?
How does the EPR reading behave in reverse, and in reheat modes?

One will be the P2 (pressure) sensor, the other will be T2 (temperature)

This could be improved by explaining the purpose of these sensors.


text:
How does the ICAO VOR separation apply to reciprocal radials?
If two aircraft are on VOR radials that are apart by 15 degrees or more and one aircraft is at least 15 DME out, they are considered laterally separate. I read this rule in ICAO documents but my question is what if radials are 180 degrees apart and one aircraft is 15 DME out? Would they still be considered separate? I don't think so because they would be reciprocal to each other in this case. Then what are the limitations of above mentioned VOR separation rule?

My question is different. Let me explain. If one aircraft is moving towards vor on radial 90  and other is also moving towards vor on radial 270, both are maintaining FL360 and first aircraft is 15 NM away from vor, r they separate?

In that case, they're (at the moment) 15+ miles apart, so they are separated. Obviously, they won't be for very much longer...


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What purpose does a control area really serve related to managing air traffic flow?
I can understand that we have control zones at busy airports to streamline the flow of landing and departing traffic. But in Control areas of many airports i have seen that routes go straight through the control area upto control zone. I mean no noticeable change occurs in route geometry. I can't understand what purpose are these control areas serving the air traffic services personnel? In other words, are'nt control zones alone enough to streamline landing and departing traffic at any airport? If not, why not? I am really confused about this.

By “control areas” do you mean TMA/TCAs? And by “routes” do you mean airways? Are you aware of SIDs and STARs? There’s a lot to explain here, so it would help us to know your current knowledge level.

Yes. I am aware of SIDs and STARs. Routes mean airways. And yes by control areas i mean TMA or TCAs.

And i am also well versed with airspace classification and the technical terms used in air traffic control.

ATC’s most fundamental job is to “separate” aircraft, i.e. keep them from hitting each other. The airspace where they are able to provide this service is called “controlled airspace”, and it can be divided into three basic types:
Control Zones designate the airspace controlled by the Tower, which is just a few miles wide and a few thousand feet high to cover the airspace they can see (in good weather), i.e. the traffic pattern.
Airways, in contrast, exist to protect en route traffic up high (far above any Control Zone) and mostly form straight lines between VORs. Since the most common place to put a VOR is at a major airport, though, the airways will appear to go directly to a Control Zone. But they actually go over the Control Zone. Note that many airways do not have complete radar coverage, so separation may be done procedurally.
Control Areas (TCA/TMA) are the bridge between the two. They are generally an inverted wedding cake around a radar site, and the most common place to put radar is at a major airport. Their job is to allow ATC to protect aircraft moving vertically by vectoring them (or through the use of SIDs/STARs) around each other laterally, and to exclude other low-flying aircraft if needed due to traffic levels.
Any airspace outside these three (which is most of the planet) is usually uncontrolled airspace, where means aircraft must depend on Big Sky Theory to protect them, because ATC can’t.

Sir as far as i know SIDs and STARs end and start at boundary points of control zone respectively. Also aircraft are often taken off the airways outside the CTAs and TMAs when required. Then what purpose does a CTA serve? If u could explain with an example of some traffic situation it would be highly appreciated.

The main purpose of controlled airspace is NOT to "streamline" traffic, but to protect traffic. Control areas are not there to "service air traffic service personnel", but to enforce certain rules upon the pilots.
In and around busy airports, the density of aircraft in the sky is relatively high, meaning there would be a relative high risk of collision if everyone were just flying around randomly. To mitigate this risk, various types of controlled airspace is established (control zones, terminal areas, control areas etc.)
Within controlled airspace, aircraft are subject to an ATC clearance, which more or less eliminates any risk of collision. Crucially, outside of controlled airspace, pilots can, essentially, fly pretty much however they want, and the only way to not hit another aircraft is if you happen to see it out the window.
Control areas are not just established around airports, but in any area where the density of air traffic is high. Typically, this means pretty much everywhere at higher levels, and around major airports at lower levels.

I agree with the purpose of control areas you described. But i couldn't find the information i am looking for in your answer. Suppose we have a control zone then what's the need for establishing a control area? I am well aware of airspace classes and air traffic control. So feel free to use technical terms.

A control zone, by definition, always extends from the ground up. A control area extends from a specified lower level and up. So the difference is that a control area does not touch the ground.


text:
Why would a plane be allowed to fly to LAX but not Maui?
While waiting for my plane in the Phoenix Airport, we get told that we're being swapped to a new plane because that plane was supposed to fly to Maui, but can't.
When asked why it would be fine to fly to LAX but not Maui, the response amounted to "Federal law prohibits us from telling you".
That plane, we were then told, had too much fuel to fly to LAX, so we ended up getting a third plane.
What sort of issue would prevent a flight to Maui but not LAX, that they would be unable (or unwilling) to tell us about?

Is it possible the paraphrased quotation was more "Federal law prohibits the plane from flying over the ocean, but the details are too long to tell you here and now."   ?

Probably nothing to do with it, but there was a NOTAM for the launch of Firefly Alpha from Vandenburg that affected flights to and from Hawaii. Original launch was scheduled for July 1. THE TEMPORARY ROUTES DESCRIBED ABV ARE FOR ALL ACFT EQUIPPED
WITH RNP-10 NAVIGATION SYSTEMS OR BETTER.  ALL OTHERS SHOULD FLIGHT
PLAN TO AVOID THE AIRSPACE

@Criggie: unfortunately, I wasn't the one who talked to them myself, so I'm paraphrasing secondhand.  Possibly? But less likely, I think.

Most likely the plane scheduled for the Phoenix (PHX) to Maui (OGG) flight had a technical issue so they needed to find a replacement. The plane was then "handed over to your route". It may very well be that your original plane went to Maui.
As you can see the PHX-OGG route is mostly over the pacific:

Planes operating such routes need to be Extended-range Twin-engine Operations (ETOPS) certified. The problem with the plane made it non-ETOPS compliant, but it could still be  safely operated on routes that have alternate airports close by. ETOPS requirements are extremely strict, so while the the PHX-OGG was out of the question, the PHX-LAX was totally fine.
Except for the fuel: the plane was fueled for 2500nm +reserves, and the PHX-LAX would only need a fraction of that. Ferrying fuel is very expensive, and while it would've been possible to offload fuel, that would also have been costly and maybe there was no time for that. Also, as Busdriver mentioned in comments, the plane might have been overweight for landing at LAX if no fuel was offloaded.
As for the reason they did not tell you the actual reason, I can only speculate that they told a little white lie, or that plane allocations and operative information such as this is actually (mildly) classified.
Link for Great Circle Mapper map of routes

Price of the extra fuel burned is negligent compared to the price of delay required to arrange a third jet and to mange additional delays to the network. The most likely reason is that plane was so fully fueled in anticipation for a longer flight that it would have been overweight in landing after a planned, shorter flight.

@busdriver the MLW scenario depends huuugely on the aircraft in question, but your point of delay cost is very good.

An example of a failure that would rule out ETOPS flight but not PHX-LAX would be a defective life raft: not exactly something you're going to need if the plane makes a forced landing in the middle of a desert.

RE your last paragraph: https://aviation.stackexchange.com/questions/55475/what-do-flight-attendants-do-if-a-passenger-asks-what-is-going-on-in-case-of-a-p

Thanks @gparyani! The answers to that question do leave the legal side of the matter unanswered, maybe a new question there...

Re: can't tell you This is speculative but they might be a little bit more cautious after the recent NTSB smackdown of Boeing sharing information that was already public but sharing it in a way that linked it to an ongoing investigation. Even if that wouldn't apply here they may have tighter guidelines to not tell anyone anything ever.

It is exceptionally unlikely that the reason a plane was not able to operate an ETOPS route would be classified at any level. Very-little-to-no information pertaining to the normal operation of a passenger airline is actually classified. A rather small percentage of airline employees would even have an active security clearance and the vast majority of those would just be people such as pilots who are still reserve members of the military (e.g. pilots who are still in the Air Force or Navy Reserves.) ETOPS regulations are openly published by the federal government itself.

@reirab I agree,  I may have used the word "classified" a bit lightly, I was thinking more along the lines of "not to be discussed with passengers" due to company policy, not NSA or CIA kinda classified.

In addition to ETOPS-related issues, it could also have to do with flights over water. I am not an expert (if someone is an expert and tells me I'm totally wrong, I'll delete this), but there are requirements for flotation devices, rafts and other safety equipment for over-water flights that are not required for over-land flights. If some of that equipment was inoperable and/or missing then it might be more cost-effective to use the already fueled plane for an over-land flight ending at an airport that can service the necessary equipment than to wait for service at the current location.
The specifics vary by aircraft model. But as one example, I found the FAA Master Minimum Equipment List for the A320 and it includes things such as:

Slide Raft Lanyards (White and/or Yellow) - (O) May be missing or damaged beyond serviceable limits provided aircraft is not operated on extended overwater flights.

Extended overwater (Part 121.339, basically lifevests and rafts instead of floating seat cushions) kicks in at 50 nm from shore. In North America, it would be pointless to have ETOPS without EOW, as you can't get more than 60 minutes away from a diversion airport without flying offshore. Some other regions, like the Himalayas and Australia, you can hit ETOPS limits over land.

@user71659 I agree. My point is that if they did an inspection before departure and found a problem (somehow) with one of the rafts then they would have to take it off of overwater flights until that was resolved but it would be fine over land.

@user71659 That's certainly true for SoCal/AZ, but not for all of North America, especially if you're flying a heavy jet with hundreds of passengers. There are plenty of parts of Northern Canada and Alaska that are more than 60 minutes from an airport that could reasonably handle a diversion for those. Fairbanks and Anchorage (and maybe Juneau? though that doesn't really matter) are probably the only airports in Alaska that could handle, say, a 777, for example. And, especially if it's winter, you'll need a place to safely house 300+ people.

@reirab:  Utqiaġvik (formerly Barrow) has a 7100-foot asphalt runway, which could probably land a 777.  Taking off again might be dicey, though.

@reirab Alaska doesn't require ETOPS, thanks to a lot of runways left over from the Cold War that were built to handle B-52s and C-5s.

@user71659 I don't think that map takes into account whether those airports actually have all of the necessary requirements to count as an ETOPS alternate for a particular aircraft, though. There are lots of runways in Alaska, but quite few of them can handle a 777, let alone have the equipment to service it and safely house its passengers in the winter. You're not likely going to be landing a 777 at Dutch Harbor any time soon, for example, let alone, say, Coldfoot. I have flown into Coldfoot and, trust me, you really don't want to land a 777 there.

@reirab Nope. This occurs regularly. ETOPS only requires a safe landing, not comfortable handling. In cases you cite, the passengers remain on board the aircraft until a replacement is flown in. Examples include the 2018 Delta diversion to Shemya, multiple diversions of 747s and 777s to Henderson Field.

@user71659 Ah, I wasn't aware of Shemya. That one makes more sense. As for 'safe' vs. 'comfort,' though, heat in Alaska in the winter quickly becomes much more the former than the latter, especially in Northern Alaska.